The value of variable-speed drives is not in the balance

07 May, 2007

No-where in the new energy centre of the Natural History Museum is there a balancing or commissioning valve. Vital Energi, which manages the centre believes that using variable-speed drives with all fans and pumps is inherently a far better approach.Initially, the centralised boiler house of the South Kensington Museum Estate, installed in 1952 in the basement of the Natural History Museum, distributed heat to a number of adjacent museums and colleges and formed a very early district-heating system. It was refurbished in 1980 with new gas-fired boilers but more recently, especially when the Science Museum and Imperial College installed CHP systems, it became clear that the NHM’s 25-year-old boiler house was over-large and inefficient and in need of refurbishment. The recently completed energy centre at the Natural History Museum is the product of an energy performance contract between Vital Energi and the NHM. It guarantees to save the museum £500 000 every year over the next 15 years. This £12 million project involves the finance, supply, installation and commissioning of the necessary plant and equipment to provide tri-generation of electrical power, heating and cooling services to both the Natural History Museum and the Victoria and Albert Museum. Through improved energy efficiency, the NHM will achieve a reduction of 1800 tonnes of carbon dioxide a year. The work involved removing two or the original four boilers, the installation of a 1.8 MW(e) gas fired CHP engine, two 750 kW absorption chillers to utilise CHP waste heat and two new cooling towers. Vital Energi will also be responsible for the operation and maintenance of the energy centre over the next 15 years. A central plank of Vital Energi’s ethos is that every aspect of the refurbishment should be engineered to function at the optimum operating point. According to Helge Wonsbek of Vital, ‘It’s all too common for systems such as this to be over-engineered “to be on the safe side”. We don’t regard that as engineering, and we take great care to size every aspect of the system to meet the exact needs of the building.’ Maintaining the balance In a complex the size of the NHM, however carefully the building-services system is designed, the various air and water circuits and sub-circuits will require to be balanced and some method of control provided to maintain that balance — otherwise the hot water and cooled air would not be delivered to the appropriate parts of the building. Traditionally, proportional balancing was achieved using balancing valves in the water circuits and balancing dampers in the air ducts to control flow by creating a restriction in the flow. In themselves these are costly devices and control is not precise. Introducing pressure drops to control flow simply wastes energy at the fan or the pump overcoming that resistance — a loss that continues throughout the life of the system. An additional complication is introduced if thermostatic radiator valves are employed, as these upset the balance of the system as they open and close. To counter this, differential-pressure-control valves are used to ensure constant system resistance — entailing greater expense. With the ever-increasing cost effectiveness of variable-speed drives (VSDs), inefficient proportional balancing with valves has increasingly given way to accurate more efficient flow control by varying the speeds of the fans and pumps delivering the air and water throughout the building. As energy costs have risen, so the cost per kilowatt of VSDs has steadily reduced and the economic case for VSD control has become undeniable. The ease of making interconnections with building-management systems is another major benefit, simplifying control and enabling energy monitoring at a central location. It is for that reason that nowhere on this extensive system has Vital Energi used regulating valves, but has instead installed Grundfos variable-speed pumps and Danfoss VSDs on all primary and secondary water circuits. As Mr Wonsbek puts it, ‘There is little point in refurbishing a system to improve its efficiency, then to install control valves which would merely drag the overall efficiency down again. Even where there is no need for variable-flow control, we have used variable-speed pumps to tune the system initially, and these will then run fixed at that reduced speed set point, so they are not connected into the building-management system. We did not do any specific cost justification calculations on the benefits of VSD on this system. We proved the economic benefits to ourselves long ago, and now VSDs are standard fitments on all our new CHP systems. They are much more cost effective than control valves. The only time we fit a valve today is for isolation purposes.’ Those parts of the system that demand control, for example the main circulation pumps, distributing hot water throughout the complex and the CHP inlet and outlet ventilation fans, are also speed controlled — as are the fans in the cooling tower. Based on past experience, Vital Energi selected Danfoss VLT 6000 series drives of 2 to 30 kW for these functions since they tied in readily to the existing BMS and offer PID control as standard. Value-engineering benefits In Vital Energi’s experience, the value-engineering benefits of VSDs are many. In addition to providing highly accurate and energy-efficient control over the fans and pumps in heating and cooling systems, modern drives enable the motors powering those fans and pumps to operate at optimum efficiency by controlling them to match the demand of load and speed at every point on the load/speed curve. Not only is energy saved by accurate control of flow, but it is also saved within the motors themselves. Fan noise is also reduced dramatically at lower speeds, motor power factor is improved almost to unity and routine maintenance is virtually eliminated. Today, inverter speed control is the proven cost effective solution. The inexorable rise in energy costs means that experienced companies like Vital Energi no longer need bother with a cost effectiveness calculation before deciding to use VSDs. The life-cost cycle has been well proven and in some cases is as short as a few months.